How to create a custom WordPress Rest Api route. https://since1979.dev/snippet-005-simple-custom-rest-api-route/

snippet-004-ajax-call-to-endpoint.js

/**
 * @see https://since1979.dev/snippet-005-simple-custom-rest-api-route/
 *
 * @uses $.ajax() https://api.jquery.com/jquery.ajax/
 * @uses console.log() https://developer.mozilla.org/en-US/docs/Web/API/Console/log
 */
$(document).ready(function () {
    let post = 9;
    let meta = '_thumbnail_id';
    $.ajax("/wp-json/mytheme/v1/meta/" + post + '/' + meta, 'get').success((data) => {
        console.log(data.meta);
    }).error((error) => {
        console.log(error.responseJSON.message);
    });
})